Tahini (or tahina) is simply a seed butter made with sesame seeds and oil. In principal it safe to tahini during pregnancy. However, there is a risk of bacterial contamination due to its high moisture content, similar to hummus. Some research suggests that toasting sesame seeds reduces the risk of contamination, however it is not definite. Hence, it is best to use your judgement to buy freshly made tahini from a hygienic facility or if you want to be completely sure, make your own tahini from sesame seeds. The recipe takes only a few minutes in a blender.
The most common use of tahina is in making hummus. However, it is used in many Middle Eastern recipes. When eating hummus or other tahina based dips, be careful of the risk of cross contamination with different foods and different people dipping in and out. It is better to avoid sharing dips.
Check out these simple recipes for hummus and tahini from this Turkish kitchen: